Background information is disclosed in the following book, articles and patent:
1. C-N. Chen and D. I. Hoult, "Biomedical Magnetic Resonance Technology", Adam Hilger, Bristol, U. K., 1989. PA1 2. D. I. Hoult and R. Deslauriers, "A High Sensitivity, High B.sub.1 Homogeneity Probe for Quantitation of Metabolites", Magnetic Resonance in Medicine, 16, 411, 1990. PA1 3. D. I. Hoult, "Audio Interpolating Phase Shifter", Electronic Design, 44125, 115, December, 1996. PA1 4. U.S. Pat. No. 4,890,062 (Haragashira, issued Dec. 26th, 1989). PA1 providing a source of a high frequency oscillating current, the source having a required characteristic impedance; PA1 providing a probe to be connected to the source for generating an electromagnetic field in response to the current, the probe having an impedance different from said required characteristic impedance; PA1 providing as part of the probe an adjustment circuit for adjusting the impedance of the probe to the required characteristic impedance; PA1 providing in the adjustment circuit a first adjustable element and a second adjustable element arranged such that adjustment of the first and second elements causes adjustment of both a resistive component and a reactive component of the impedance; PA1 arranging the first and second elements such that, over at least a part of a range of adjustment of the first and second elements, adjustment of the first element while the second element remains constant generates a change in the impedance which is substantially orthogonal to a change in the impedance obtained by adjustment of the second element while the first element remains constant and such that adjustment of the first element to a first required adjustment position and adjustment of the second element to a second required adjustment position causes the impedance of the probe to be adjusted to said required characteristic impedance; PA1 providing a detection circuit responsive to the impedance of the probe and causing the detection circuit to provide a first and a second separate output values which are orthogonal to one another and which are indicative of a difference in the impedance of the probe from said required characteristic impedance; PA1 arranging the output values such that the first is indicative of a difference of the first element from the first required adjustment position and such that the second is indicative of a difference of the second element from the second required adjustment position; PA1 and using the first and second output values for controlling respectively the first and second elements.
The present invention is particularly but not exclusively concerned with transforming the impedance of a probe of a magnetic resonance imaging system so that the impedance of the probe matches the required characteristic impedance of the source of radio frequency current used to power the probe.
Nuclear magnetic resonance imaging (MRI) has become one of the most powerful non-invasive diagnostic tools available. In MRI a person is placed within a strong magnetic field so that the nuclei in the body become aligned with the field. The transient application of a second, albeit alternating, magnetic field, this time in a direction perpendicular to the first field, causes the nuclei to be thrown out of alignment and consequently to precess, thereby producing a signal used in imaging. The brief application of the second field is accomplished by putting a large radio frequency (r.f.) alternating current through a set of coils placed about, or close to, the person. Generally, although not necessarily, the same coils are used to receive by electromagnetic induction the signal from the precessing nuclei.
One of the essential prerequisites for successful operation of the MRI system is that the impedance of the coils (very roughly of the order of 75 ohm reactance and 1 ohm resistance) be transformed so as to equal the characteristic impedance of the source of the r.f. current (typically 50 ohm resistive via the intermediary of a connecting coaxial cable having a characteristic impedance that is also 50 ohm), thereby maximizing the current flow in the coils. As the coil impedance is complex, comprising both reactance and resistance, and most importantly, may change slightly (both in reactance and resistance) as a patient is inserted in the coil or indeed from person to person according to the latter's size and electrical conductivity, two variables are needed to transform the impedance to the desired value. Thus, in one well-known implementation, two variable capacitors are used. The first is placed in parallel with the coils, thereby causing the ensemble to resonate at a frequency close to the magnetic resonance frequency. This capacitor is commonly referred to as the "tuning" capacitor. The second variable capacitor, commonly known as the "matching" capacitor, is connected between one end of the tuning capacitor and an input terminal. A connection is also made between the other end of the tuning capacitor and a second input terminal ("ground"). Tedious adjustment of the two capacitors in a repetitive manner then allows the input impedance of the assembly or "probe", as measured between the two terminals, to be set to the desired value. Some skill is needed in this process as the adjustment of one of the capacitors varies the effect of the other; in short the adjustments are interdependent.
It should be noted that without this process of "tuning and matching" i.e. impedance transformation, both stimulation of the magnetic resonance phenomenon and reception of the ensuing signal are poor. It should be further noted that several well-known variants of this impedance transformation scheme exist, including: splitting the matching capacitance in two and placing two variable ganged capacitors between the ends of the tuning capacitor and the terminals; placing matching and tuning capacitors in series with the coils and one another and connecting the current source across the matching capacitor; the latter configuration with the tuning capacitance split into two and two ganged capacitors placed in series and on either side of the matching capacitor; etc. Further details may be found in the book by Chen and Hoult. However, all such schemes have the same goal--to transform appropriately the impedance of the coils with the aid of two or more variable capacitors.
The tedium of tuning and matching has existed for many years and has received attention at various times over that period with no practical solution becoming available for assisting or automating this adjustment procedure. Until now, therefore, the adjustment procedure has relied upon the skill of the technician in providing the necessary manual adjustments of the two (or more) capacitors to obtain the required characteristic impedance.
One attempt to automate this procedure is set out in U.S. Pat. No. 4,890,062 (Haragashira--Issued Dec. 26, 1989). This patent discloses a technique in which a detector is used to obtain voltages proportional to the resistive (real) and reactive (imaginary) components of the impedance. These signals are then arranged to actuate a controller which supplies a control signal to change appropriately the tuning and matching capacitors. However, the technique proposed is complex and has not led to any commercial operation. Further, it may be shown to be ineffective for certain combinations of tuning and matching capacitance. In effect the technique has been abandoned.